Photoimageable compositions are used for various purposes in the manufacture of printed circuit boards of the like. Photoimageable compositions in the form of a primary imaging photoresist are used in the formation of the printed circuitry itself. In a typical procedure, a panel for forming a printed circuit board comprises a thin layer of metal covering a non-conductive substrate. A layer of photoresist is applied over the metal layer. The photoresist is exposed to patterned actinic radiation, e.g., by passing illumination through artwork having an opaque/transparent pattern. The exposed photoresist is then developed by exposure to a solution or solvent which, depending upon the type of photoresist, washes away either the exposed or unexposed portion of the photoresist, leaving a patterned layer of either unexposed or exposed photoresist. The panel can then be etched with a solution which removes the metal layer from those portions of the plate from which photoresist has been removed. Subsequent to etching, the remaining photoresist layer is generally stripped away. Alternatively the panel can be pattern plated, whereby the areas devoid of photoresist are electrolessly or electro-chemically plated with copper or the like, the resist is stripped, as above-mentioned, and the newly exposed areas are then removed by etching, leaving behind the pattern plated areas.
Other photoimageable compositions are used for forming solder masks in a similar manner. These compositions are used to form a hard, permanent layer which overlies printed circuitry except in those portions of the printed circuitry to which solder is to be applied.
Photoimageable compositions may be applied to a panel in liquid form and then either allowed to dry or to partially cure so as to form a semi-stable layer. Alternatively, the photoimageable composition may be used to form a dry film comprising a layer of photoimageable composition on a cover sheet of support material. The layer of photoimageable material is subsequently transferred from the support sheet to a surface of the plate.
There are a number of advantages to applying a photoimageable layer to a circuit board from a dry film rather than as a liquid. In particular, dry films are free of organic solvent and therefore eliminate solvent hazards from the workplace and eliminate the need for apparatus to protect the immediate work environment and the more general environment from organic solvent emissions.
Dry films comprise the layer of photoimageable material and the cover sheet of support material. The support material is somewhat flexible but has sufficient rigidity to provide structure to the layer of photoimageable material. Typically the cover sheet is formed of a polyester, such as polyethylene terephthalate (PET), such as that sold as MELINEX.RTM.. As the photoimageable layer is to be transferred from the cover sheet, a release agent may be applied to the surface of the cover sheet which carries the photoresist layer. It is typical to also provide a protective sheet, e.g., a sheet of polyethylene, on the surface of the photoimageable layer opposite the cover sheet. The protective sheet, in addition to protecting the photoimageable layer, better enables the dry film to be rolled into a reel. The protective sheet is removed from the photoresist layer prior to application of the dry film to the circuit board U.S. Pat. No. 4,530,896 and U.S patent application Ser. No. 07/210,194 filed June 20, 1988 the teachings of each being incorporated herein by reference, describe dry films in which a top coat is interposed between the cover sheet and photoimageable layer, which top coat is selectively adherent to the photoimageable layer (relative to its adherence to the cover sheet). The top coat serves to protect the photoimageable layer from mechanical damage and from oxygen after removal of the cover sheet prior to exposure to patterned actinic radiation. Other examples of dry films can be found in U.S. Pat. Nos. 3,887,450, 4,539,286 and 4,610,951, the teachings of each being incorporated herein by reference.
The present invention is generally directed to photoimageable compositions and dry films formed therefrom which are used in the manufacture of printed circuit boards, including those in which the photoimageable composition is a primary imaging photoresist and those in which the photoimageable composition is a solder mask-forming composition. The invention is applicable to dry films in which the photoimageable layer is in direct contact with the cover sheet (in which case the photoimageable layer is generally exposed through the cover sheet) and those in which a top coat is interposed between the photoimageable layer and the cover sheet (in which case the cover sheet is generally removed prior to exposure of the photoimageable layer). In addition to use in the manufacture of printed circuit boards, dry films having photoresist layers are used in chemical milling, lithographic printing gravure printing and the like, and the invention is applicable to photoimageable compositions and dry films formed therefrom regardless of the application of the dry film.
As explained above, the photoimageable layer of a dry film is sandwiched on a supportive cover sheet and usually between the cover sheet and a protective sheet. The manufacture of the dry film includes the winding of the multi-layered material around a carrier core. This creates internal tensions and pressures, whereby the highly viscous, semi-solid photoimageable composition can be forced to relieve those pressures by oozing out along the sides of the coil, a phenomena known as cold flow. Cold flow causes serious problems, such as edge fusion and chipping of the photoimageable layer during unwinding. Furthermore, cold flow contributes to internal non-uniformity of the thickness of the photoimageable layer.
Various methods have been devised to control cold flow. These include flash photolysis to harden the ends of the coil and thereby produce a damming effect, application of an adhesive end-capping device to stem the flow, use of embossed cover sheets to physically relieve the internal pressures, and use of amphoteric interpolymers in the photoimageable compositions to reduce the fluidity of the photoimageable compositions.
The present invention is specifically directed to photoimageable compositions used to form dry films; the photoimageable compositions containing a chemical which acts to reduce cold flow of the photoimageable layer.